Genesis 5 clearly shows the number of years people lived from Adam to Noah. In the modern world, we know that 1 year equals to the number of days the earth revolves around the sun, but this knowledge was just claimed in the 17th century by Galileo while the book of Genesis should have been written much earlier.

As another point of view, the calendar system that most parts of the world use today dated back to 45 BC through the introduction of Julian calendar. This may be the first time a wide group of society agreed on how long 1 year is.

How did the author of Genesis define how long 1 year was?


While Why did people live so long before the Flood concerned about the ages (specific to Genesis 5), my question wonders more on the way a year was defined in general, possibly valid across the Pentateuch, by giving Genesis 5 as one example. It does not ask why men lived with such long ages, which had answers such as: bodily well-being was enhanced; a greater degree of nourishment; water before the flood protected human ages; etc. These answers in that discussion did not address my question.

Hence my question could have been, "How did Moses define 1 year in Pentateuch?" The problem with this phrase is that it could mistakenly viewed as a duplicate with other discussions, such as who wrote the Pentateuch, or whether the Pentateuch was written by the same author.

In contrast, my question emphasizes the definition of a "year" written across the Pentateuch. To avoid conflicting discussions, herewith I specify one example by pointing to Genesis 5. An example of satisfying explanation is from one comment below, that one didn't need a calendar to know the years, but used winter and summer.

  • Welcome to Christianity.SE. For a quick overview of what this site is about, please take the Site Tour. – Lee Woofenden Nov 22 '15 at 8:17
  • This might be more about astronomy than about religion. – Matt Gutting Nov 22 '15 at 13:02
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    Just a small point: most parts of the world now use a solar calendar. Even, 800-900 BCE, the period we are probably talking about, you didn't need a calendar to know the years. You had winter and summer. – Dick Harfield Nov 22 '15 at 19:52
  • Thanks. I erased the word 'lunisolar' from the body above. – Curioso Nov 22 '15 at 23:05
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    This is an interesting question but I think it probably belongs in biblical hermeneutics. – Ben Mordecai Nov 23 '15 at 2:31

From the point of the Exodus at least the Hebrews used a lunar calendar for tracking holidays and festivals. This is why there are some historical Christian controversies over the dating of Easter, since the Passover can move throughout the solar year and land on non-Lord's Days. Do you follow the Jewish passover or set a permanent date?

Since a lunar calendar drifts a few days every solar year, the months gradually correspond to different seasons of the year. A lunar year has roughly 355 days, while a solar has roughly 365.

Either way, the extremely long lifetimes of some of the characters in Genesis are not explained by the plus or minus 10 days per year.

Nonetheless, the author of Genesis (or the editors of the canonical edition) are clearly doing something interesting with the use of ages and years. Consider these two passages:

These are the generations of Shem. When Shem was 100 years old, he fathered Arpachshad two years after the flood. And Shem lived after he fathered Arpachshad 500 years and had other sons and daughters. When Arpachshad had lived 35 years, he fathered Shelah. And Arpachshad lived after he fathered Shelah 403 years and had other sons and daughters. When Shelah had lived 30 years, he fathered Eber. And Shelah lived after he fathered Eber 403 years and had other sons and daughters. When Eber had lived 34 years, he fathered Peleg. And Eber lived after he fathered Peleg 430 years and had other sons and daughters. When Peleg had lived 30 years, he fathered Reu. And Peleg lived after he fathered Reu 209 years and had other sons and daughters. When Reu had lived 32 years, he fathered Serug. And Reu lived after he fathered Serug 207 years and had other sons and daughters. When Serug had lived 30 years, he fathered Nahor. And Serug lived after he fathered Nahor 200 years and had other sons and daughters. When Nahor had lived 29 years, he fathered Terah. And Nahor lived after he fathered Terah 119 years and had other sons and daughters. When Terah had lived 70 years, he fathered Abram, Nahor, and Haran. (Genesis 11:10-26 ESV)


And God said to Abraham, “As for Sarai your wife, you shall not call her name Sarai, but Sarah shall be her name. I will bless her, and moreover, I will give you a son by her. I will bless her, and she shall become nations; kings of peoples shall come from her.” Then Abraham fell on his face and laughed and said to himself, “Shall a child be born to a man who is a hundred years old? Shall Sarah, who is ninety years old, bear a child?” (Genesis 17:15-17 ESV)

What is happening here? We have a man named Abraham who was himself the child of a 70-year old, finding it laughably remarkable that he, as a 100-year old is told he will have a child. In Genesis 5, we are told of people who had children at 130 years, 105 years, 162 years, 187 years, and 182 years. In fact, according to the record Abraham himself would have been outlived by Shem, his own great, great, great, great, great, great, great, grandfather! Yet it is critical to the story that as readers we recognize that Abraham is not crazy to think that old people having babies is not normal.

In the Ancient Near Eastern context, it's fairly common for kings and patriarchs to be assigned superhuman ages often as a sign of greatness, but probably not with the intention of providing a historical record of their biological lifespan. So to exegete the passage, we need to go after the intended meaning according to the expectations and conventions of the genre. It's not 100% clear what those expectations are, but it is clear that the long lifespans in the genealogies are probably not talking about people who actually lived to superhuman ages.

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  • While your points are reasonable, note that it's absolutely unremarkable for a 30 year old woman to give birth, not out of the ordinary for a 40 year old woman, getting quite rare for a 50 year old, and almost unheard of for a 60 year old. So it's not unbelievable that in the same way there could have been a big difference in fertility between a 70 year old and a 100 year old in Abraham's time. (This is all for women however; even now men remain fertile much longer.) – curiousdannii Apr 15 '19 at 13:08

It is hugely wrong to suppose that there existed poor levels of sophistication in the ancient civilisations before the Roman/Greek era especially when it comes to calculations of the length of the year.

If we accept the Bible's own testimony, then the Pentateuch was written by Moses who died 1406 BC, having been very well educated in Egypt.

At the time of Moses the Egyptians used a 365 day, with no extra days to make up for discrepancies. (So New Year's Day would shift about a quarter of a day each year and circle round and back to the same day of the solar year after 1508 years.)

The Egyptians used the 365 day year for so long that New Year’s Day drifted round the solar year to cycle back to the starting point almost 3 times. They used it for almost 4500 years. https://pumas.jpl.nasa.gov/files/04_21_97_1.pdf

During the 40 years of wandering in the wilderness (Leviticus 18:3) after the Exodus, and more specifically when at the foot of Mount Sinai (Leviticus 27:34), Moses wrote Leviticus, in which was instituted a lunar/solar year. The Israelites followed a lunar calendar of 12 lunar months (354 days) but (the High Priest) added an extra lunar month to the length of the year if the last harvest of the autumn had not been gathered in by the fifteenth day of month 7 (Leviticus 23:39). So the Israelites between the time of Moses and the Captivity (about 600 BC) were not taken up with attempts to measure the length of the year by astronomical observations, in the way that other cultures around them were, but rather were guided by the timing of the last harvest of the agricultural year in the autumn. What they did observe was the moon to decide when each month began.

In practice obedience to Leviticus 23:39 would have resulted in the addition of 7 months in a 19 year cycle. The extra month could have been an additional month 7 (Tishri) so in a year where a month was added there would have been Tishri 1 and Tishri 2. Or, more likely, it would have been an additional month 6 where the High Priest would anticipate whether the harvest could be fully gathered in before the set date (15th of month 7), and if not he would add an extra month 6 (Elul) making Elul 1 and Elul 2.

So 12 of the years in the 19 year cycle had about 354 days, and 7 of the 19 years had about 384 days.

(A lunar month is 29.53059 days, so 19*12 lunar months and 7 lunar months is 6939.69 days. And 19 solar years is 365.2421875 * 19 = 6939.6 days.)

By the time of the end of the Babylonian Empire (539 BC) a system of adding 7 lunar months every 19 years had been formalised and adopted by the Babylonians to keep the year in step with the solar year to err by about 129 minutes every 19 years. ((A vast improvement on the Egyptian calendar which erred by about 4.5 days over 19 years.)) Thereafter it was adopted by the Jews from the Babylonians. For religious purposes the Jews still follow the same calendar. https://en.wikipedia.org/wiki/Hebrew_calendar

The 19 year cycle of the Babylonian calendar was itself following the calendar of the Sumerians from at least as far back as 2000 BC.

It is quite possible that some agricultural societies, well before the time of Moses, measured a year to be the length of time between one annual harvest and the previous one.

The question about the book of Genesis is how did Moses produce it? Did he receive all the information by a direct revelation from the Lord, or did he receive all or some of the information from previous sources (which we can assume were themselves writing under divine inspiration). If he received them from inspired sources then we would need to know the length of a year understood by those original sources.

In addition to this we do not know if conditions before the Flood were precisely the same as they are now. God could have used a natural cause to bring about the Flood which could have had other consequences as well such as the earth's distance from the sun and the orbit of the planet. Or God could have judged mankind with other changes to the planet by a direct Divine act. We do not know, unless Scripture tells us, if the earth's orbit and the length of the year were different before the Flood, though it is difficult to imagine such a difference would be anything more than a few days per year.

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Is it possible that the word 'year' has been either misunderstood or even mistranslated? It is clear that it refers to a distinct period of time. Although a year was a distinct period of time, the lunar month was also paramount, as it determined the Sabbaths that they observed, with a new or full moon being mentioned in many verses within the Pentateuch....as its showed the ancient Hebrews the beginning of every months and also every seventh day sabbath. As pointed out in Deuteronomy 5:10 - we must still observe the 7th day sabbath! It has been worked out that, for example, Adam lived for 930 'years' - when determined each year to be a lunar month, his age will actually have been in his 75th solar year! Methuselah lived until he was 969 'years' old - which means he lived into his late eighties. I've seen a Web site, written by John MacKay that determine this....as its far more realistic, I think it should be considered!

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  • Hello Peter, your answer is incomplete but it's a good start. Answers need to be supported, rather than one's onw opinion or view. Since you cite the Theologian John MacKay (I edited in a link to his works to clarify which person you were taking about) what you need to do to support you answer is to summarize what he was referring to and then identify which of his works / essays / tracts held those points. Welcome to Christianity.SE. Please take the tour and visit the help to see how an SE site works differently than a forum. – KorvinStarmast Apr 15 '19 at 12:46
  • If you want to say that Genesis 5 is using "years" to mean months, then you'd have to conclude that it says that the patriarchs became fathers as young as 5 years old. Which is ridiculous. Such "analysis" really has nothing going for it, other than people's unwillingness to consider anything beyond what is normal now. – curiousdannii Apr 15 '19 at 13:02

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